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Charge, in Electricity. A body may be electrified in various ways; that is. it may be brought to a condition in which it exhibits certain electrical properties. The body is then said to possess a charge of electricity, this expression to be understood as meaning the amount of electrification. It does not in itself imply that electricity is a form of matter, a subtle fluid permeating all matter and filling all free space, or a modification of the ether of the physicists, though certain of the properties of an electric charge may be in part explained by any one of these hypotheses. It states, however, that electricity may be measured. A charge may be positive or negative: a positive charge is similar to that produced on glass by rubbing it with silk; a negative charge is similar to that produced on sealing-wax by rubbing it with fur. Like charges repel each other, unlike charges attract. Electrical properties are only observable when one kind of electricity preponderates; and inasmuch as the normal state of things in a system of conductors is an exact neutralisation of the two kinds, the isolation of a charge of one kind in a body must involve the isolation of an equal amount of the opposite kind elsewhere. A charged, insulated, conducting sphere, for instance, placed in the middle of a room causes the accumulation of a charge of equal amount and of opposite kind on the inner surface of the walls of the room. This induced charge as it is called will remain bound so long as the insulated sphere retains its charge. And further, since unlike electricities attract each other, it follows that the charge on the insulated sphere must reside on its surface, a property common to all charged conductors. The insulating medium separating two charges has to oppose their tendency to unite or to separate still further, and is therefore subjected to strain. That it is thus strained has been made manifest in various ways. If the strain be too great the continuity of the insulator will be broken and the disposition of the charges will be changed. The distribution on a body depends on its shape and on the position and shape of the neighbouring bodies. It is most intense in those regions on the surface of the body where the curvature is greatest. Hence the readiness of discharge at sharp points. An electrical charge admits of measurement, for the attractive or repulsive force exerted by two separate charges depends on their respective magnitudes and may be directly measured. The unit quantity is called the coulomb, and is such that if concentrated on a small conductor at a centimetre distance from an equally charged small conductor, a repulsive force of one dyne will tend to increase their distance apart.